A plasma display panel (hereinafter also referred to as “PDP”) is suitable for displaying a high-quality television image on a large screen. Thus, there has been an increasing need for various kinds of display devices using the plasma display panel.
The PDP comprises a front panel and a rear panel opposed to each other. The front panel and the rear panel are sealed along their peripheries by a sealing material. Between the front panel and the rear panel, there is formed a discharge space filled with a discharge gas (helium, neon or the like).
The front panel is generally provided with a glass substrate, display electrodes (each of which comprises a scan electrode and a sustain electrode), a dielectric layer and a protective layer. Specifically, (i) on one of principal surfaces of the glass substrate, the display electrodes are formed in a form of stripes; (ii) the dielectric layer is formed on the principal surface of the glass substrate so as to cover the display electrodes; and (iii) the protective layer is formed on the dielectric layer so as to protect the dielectric layer.
The rear panel is generally provided with a glass substrate, address electrodes, a dielectric layer, partition walls and phosphor layers (i.e. red, green and blue fluorescent layers). Specifically, (i) on one of principal surfaces of the glass substrate, the address electrodes are formed in a form of stripes; (ii) the dielectric layer is formed on the principal surface of the glass substrate so as to cover the address electrodes; (iii) a plurality of partition walls are formed on the dielectric layer at equal intervals; and (iv) the phosphor layers are formed on the dielectric layer such that each of them is located between the adjacent partition walls.
In the PDP, the display electrode and the address electrode perpendicularly intersect with each other, and such intersection portion serves as a discharge cell. A plurality of discharge cells are arranged in the form of a matrix. Three discharge cells, which have red, green and blue phosphor layers in the arranged direction of the display electrodes, serve as picture elements for color display. In operation of the PDP, ultraviolet rays are generated in the discharge cell upon applying a voltage, and thereby the phosphor layers capable of emitting different visible lights are excited. As a result, the excited phosphor layers respectively emit lights in red, green and blue colors, which will lead to an achievement of a full-color display.
Recently, miniaturization of the discharge cells has been promoted by a demand for a higher definition of the PDP. However, a size reduction of the discharge cells leads to a decrease in emission brightness and thus an increase in power consumption. This is caused by a decrease in an opening ratio, a decrease in light emission time per picture element attributable to an increase in picture element number, a decrease in luminous efficiency or the like. As a method for increasing emission brightness, there has been proposed a method of increasing the opening ratio by decreasing the width of partition walls of the rear panel. However, even in this method, the emission brightness is still insufficient and a further improvement is required.
There has been proposed another method wherein a dielectric constant of a dielectric body in a front panel is decreased, and thereby reducing a reactive power upon discharge so as to improve the luminous efficiency. According to a formation of a front-sided dielectric layer in current method for producing PDPs, a dielectric material which contains glass powder with a size of several μms, an organic binder and a solvent is applied onto a glass plate by a known process such as screen printing process, die coating process or the like. Subsequently, a flat dielectric layer with high transmittancy is formed from the glass material by a drying step, a debindering step (300 to 400° C.) and a calcining step (500 to 600° C.). However, as for current dielectric materials, the glass powder tends to be melted at a low temperature, and thus a “material capable of decreasing a melting point of the glass (e.g. Bi)” is added thereto (see, for example, Japanese Patent Kokai Publication No. 2002-053342). Such material capable of decreasing a melting point of the glass has low purity and has a high dielectric constant of 10 or more. Although the dielectric constant can be decreased by adding other substances (e.g. alkali metal), a highly conductive metal such as silver is used as a main component in an electrode of PDP, and thus a diffusion and colloidization of the silver are promoted due to ion migration, which leads to an yellowing phenomenon in the dielectric body. The yellowing phenomenon has a great adverse influence on the optical characteristics of PDP.
In order to increase emission brightness by decreasing the dielectric constant of the dielectric layer, it is necessary to develop a new low dielectric constant material to replace current types of glass paste, and also develop a method of forming a dielectric layer using such material. As a process for forming a dielectric layer made of high-purity oxide, there has been a process in which a solid oxide is deposited on a substrate by sputtering process under vacuum atmosphere (i.e. sputtering deposition process), and also there has been another process in which a material is deposited by decomposing a raw material with plasma (i.e. chemical vapor deposition process). Although these processes can produce a dielectric layer with a high purity and a low dielectric constant, expensive vacuum facilities are required and a film-forming rate is so low as about several 100 nm per minute. In this regard, for preventing a dielectric breakdown phenomenon upon application of voltage, the required thickness of the dielectric film is usually 10 μm or more and thus the larger number of the equipments are required to increase a productivity thereof.
Alternatively, it has been proposed to melt silica with high purity. However, the melting of such silica is not practical since a high temperature of 1000° C. or higher is required.
As a process for forming a dielectric layer with low dielectric constant while ensuring productivity, there has been proposed a sol-gel process. According to this process, a metal alkoxide is hydrolyzed in a solvent to give a silicon compound and subsequently the silicon compound is subject to a condensation polymerization treatment by heating thereof to form a film which mainly consists of silicon oxide. For example, in a case where the silicon compound is a silicon hydroxide (Si(OH)4), a network of —Si—O—Si— is formed by the following condensation polymerization reaction and thereby a solid SiO2 is formed to give a dielectric layer.nSi(OH)4→nSiO2+2nH2O                (n: an integer of 1 or more)In a case where the silicon compound is a siloxane, a dielectric layer is formed by the following condensation polymerization reaction.        

According to the sol-gel process, a dielectric layer can be formed at a low temperature since no melting of the glass is required. However, a cracking phenomenon generally occurs in the dielectric layer as a result of a volume shrinkage thereof attributable to the condensation polymerization reaction. For this reason, it is generally difficult to form a thick film (e.g. film with about several μms). In this regard, particularly when the dielectric layer is formed over an electrode, a stress caused by volume shrinkage upon the condensation polymerization reaction is concentrated in the dielectric layer adjacent to the edge of the electrode. More specifically, a tensile stress applied to the dielectric layer by a substrate as a result of solidification attributable to the condensation polymerization reaction is concentrated adjacent to the edge of the electrode. As the display electrode of the front panel, a conductive layer mainly made of silver (i.e. bus electrode) is formed on an ITO electrode (i.e. transparent electrode) so as to decrease a resistance of the display electrode. When the dielectric layer is formed to cover the bus electrode, a stress attributable to the condensation polymerization reaction is concentrated adjacent to the edge of the electrode and thus a cracking of the dielectric layer occurs along edge of the electrode (see FIG. 6 and FIG. 7).
To cope with the cracking, there has been proposed a method for inhibiting the shrinkage by using an acid or base catalyst and a metal alkoxide with an organic functional group such as phenyl group, acryl group or the like (see, for example, Japanese Patent Kokai Publication No. 2005-108691). This method can form a thick dielectric layer. It is however possible in this method that a decomposition of the organic functional group is caused under a high-temperature atmosphere at about 400° C. This means that the cracking phenomenon may occur due to the volume shrinkage and thus it is impossible to guarantee quality in a high temperature step performed after the dielectric layer forming step.
Alternatively, there has proposed another method in which a dielectric layer is formed by calcining a dielectric material under an inert atmosphere at about 400° C. (see, for example, Japanese Patent Kohyo Publication No. 2003-518318). According to this method, the dielectric material is prepared by using a metal alkoxide with an organic functional group having a smaller molecular weight than that of the above organic functional group. Namely, the dielectric material is prepared by using a metal alkoxide with methyl group, ethyl group or the like. This method can form a layer with a thickness of 10 μm or more. However, according to this method, all steps after the dielectric layer forming step must be performed under an inert gas atmosphere, and thereby large facilities and strict control are required.
Alternatively, there has been proposed another method in which a stress release layer is formed between a dielectric material formed by a sol-gel process and a substrate (see Japanese Patent Kokai Publication No. 2007-109479). According to this method, a stress can be released in a dielectric layer by a difference in a thermal expansion coefficient, and thereby it is possible to form the dielectric layer with a small internal stress. However, the internal stress existing in the dielectric layer is not caused only by the difference in thermal expansion coefficient. Namely, most of the internal stress is due to a stress caused by the condensation polymerization reaction peculiar to the sol-gel process. Therefore, even if the stress release layer is provided, the condensation polymerization reaction may promote the cracking phenomenon to occur. Specifically, when a dielectric layer is formed by the condensation polymerization, a stress of the layer tends to be concentrated adjacent to electrode edge as a result of its volume shrinkage attributable to the reaction, and thus the cracking occurs in the dielectric layer along the edge portion of the electrode.
In this regard, it is possible that all the condensation polymerization reaction may not be completed even after the formation of the dielectric layer. In this case, when the dielectric layer is exposed to a higher temperature upon a subsequent panel sealing, the uncompleted condensation polymerization reaction proceeds and thus the cracking occurs along the edge portion of the electrode after the formation of the dielectric layer.